1. Field of the Invention
The present invention relates to processes for preparing graft copolymers by radiation induced graft polymerization of fluorostyrenic monomers, employing monomer microemulsions. The graft copolymers may be formed into membranes, including ion exchange membranes.
2. Description of the Related Art
The preparation of graft polymeric membranes by radiation induced graft polymerization of a monomer to a polymeric base film has been demonstrated for various combinations of monomers and base films. The grafting of styrene to a polymeric base film, and subsequent sulfonation of the grafted polystyrene chains has been used to prepare ion-exchange membranes.
U.S. Pat. No. 4,012,303 reports the radiation induced graft polymerization of α,β,β-trifluorostyrene (TFS) to dense polymeric base films using gamma ray co-irradiation. The graft polymerization procedure may use TFS in bulk or in solution. The '303 patent reports that aromatic compounds or halogenated compounds are suitable solvents.
U.S. Pat. No. 4,605,685 reports the graft polymerization of TFS to pre-irradiated polymeric base films. Dense polymeric base films, such as for example polyethylene and polytetrafluoroethylene, are pre-irradiated and then contacted with TFS neat or dissolved in a solvent. The '685 patent also states that the base films may have fine pores.
U.S. Pat. No. 6,225,368 reports graft polymerization of unsaturated monomers to pre-irradiated polymeric base films employing an emulsion including the monomer, and emulsifier and water. In the method of the '368 patent, a base polymer is activated by irradiation, quenched so as to affect cross-linking of the polymer, and then activated again by irradiation. The activated, cross-linked polymer is then contacted with the emulsion. Graft polymerization to dense polymeric base films is reported, although the '368 patent mentions that microporous base films may also be employed. The '368 patent also states that the use of the disclosed method eliminates homopolymerization caused by irradiation of the monomer, and that this allows the use of high concentrations of monomers in the emulsion.
These methods of preparing graft polymeric membranes have several disadvantages.
When neat TFS is employed in graft polymerization reactions, it can be difficult to achieve a contact time between the monomer and a dense irradiated base film resulting in the desired level of graft polymerization that would be suitable for high-volume production. Typically, the neat monomer does not wet the surface of the base film very effectively, and this can result in an undesirably low graft polymerization rate unless a prolonged contact time is employed. Further, the use of neat TFS may adversely increase the cost of the graft polymerization process, due to the excess of monomer that is required.
A disadvantage of graft polymerization reactions carried out using TFS solutions is the level of graft polymerization drops significantly as the concentration of monomer in the solution is lowered. Indeed, the '303 patent reports a significant decrease in percentage graft with decreasing TFS concentrations. The drop in percentage graft may be mitigated by increasing the radiation dosage and/or the grafting reaction temperature, but this necessarily increases the energy requirements of the graft polymerization process and may promote undesirable side reactions. Overall, the use of TFS in solution tends to undesirably increase the cost of the graft polymerization process.